I thought of a problem in the firmware of FX3 while the cause was hardware.I decoupled the R820T2 3V3 power supply using a separate LDO from the 5V USB bus tosolve the problem in the prototype.

The R820T can be used to receive frequency band in the range 30MHz -1800MHz. The tuner uses a clock generated by Si5351A at 32.000MHz, the REGDIV bit of reg 4 is set to 1 to divide it by two internally to the nominal 16MHz.The IF output is selected at 5MHz ( I used up to 7.5MHz) and it’s sampled by the ADC at 64Msps and then decimated down to 8Msps or less. The R820T data sheet states that the standard IF filters are implemented for 6/7/8 MHz channel bandwidths.

The LNA, the mixer and the VGA gains can be set manually although their precise values are absent from the datasheet. They can also be set automatically via automatic gain control (AGC) in order to optimize the signal to noise ratio (SNR).